Production of catalytic cracking feed stock from petroleum residues by vacuum flashing



Oct. 15, 1957 D. c. HAUscH PRODUCTION OF CATALYTIO ORAOKING FEED STOCK FROM PETROLEUM EEsIOUEs BY VACUUM FLASHING Filed July 25, 1956 wEOPPOm PRODUCTION F CATALYTIC CRACKING FEED vSTOCK FRM PETRDLEUM RESIDUES BY VAC- UUM FLASHING Douglas C. Hansch, Anacortes, Wash., assgnor to Shell Development Company, New York, N. Y., a corpora- Ation of Delaware A Application `luly 23, 1956,1Serial N0.. 599,481`

2` Claims. (Cl. 196-72) VThis invention relates to the separation and recovery of liashed distillate suitable as a feed stock for catalytic `cracking from petroleum residues by vacuum flashing.

The object of the invention is to provide anjimprove- ,ment in the vacuum flashing of petroleum residues to prepare feed stock for catalytic cracking whereby the severity of the flashing operation maybe increased there- 'by `increasing the yield of catalytic cracking feed stock of suitable quality.

In normal refinery practice crude petroleum is iirst `topped to remove gasoline and if desired other low Vboiling straight-run materials.

The residue remaining is called reduced crude.. The main feed stock for catalytic ycracking is obtained from this reduced crude and the most common method for separating this catalytic crackin'g `feed stock is by vacuum ilashing.

In essence, 'vacuum iiashingis Va'simple operation. The

-oil is heated with `partial vaporization and the mixture Yof vapors and unvaporized liquid is passed to a liash `tower under vacuum. The resulting vapors are con- Vfden'sed to give a so-called ashed distillate. This oper- Yation can be carried out without trouble provided that one-is satisfied with a relatively Vmild or shallow flash. However, difficulties arise when one strives for the deepest possible liash to obtain the maximum quantity of catalytic crackingfeed stock. The first of these is due to so-called c`:arry-over of residue with the liashed vapors. It is Well known that even quite small amounts of residue entrained `in thetlashed vapors severely degrade the quality of the lia-shed distillate as a catalytic cracking feed stock. "commercial, `operation lit is the practice to controlthe depth of flashing such that the carry-over of residue does not exceed more than about 0.1% by weight o'f .the flashed distillate produced.

`-In order to curtail vthe carry-over of residue with the distillate, trays above the hash have been used in some cases. Such trays are, however, found to 'be only partially effective and furthermore have the disadvantage -of offering an appreciable pressure drop which limits the vacuum that may be pulled in the flashing zone.

Also, inrsome cases a foraminous mistl coalescer has beenmsed in place of trays. Such mist coalescers are much more effective than trays. Preferred foraminous mist coalescers take the form ofa mat `of tangled wire several inches thick such for example, as .the so-called York mat manufactured by t-he Otto H. York Incorpojrated, West OrangeNew Jersey, or the so-.called Metex mat `manufactured by vthe Metal Textile Corporation,

'Rosella New Jersey. These mats have an open volume lnI 2,809,923 YPatented Oct. 15, 1957 necessity be wet. In order to prevent clogging it has been the practice in some cases in the past to spray a part of the condensed flashed distillate product onto the Wire mat from the top. This liquid passing down through the mat washes it of clogging material. This operation has been satisfactorily used in some cases. See for example, the Oil and Gas Journal for November 7, 1955, page l03.

ICS

As the severity of flashing is increased, however, this operation becomes impossible due to clogging of the mat even with the largest practical volume of liquid spray. Obviously as the amount of liquid spray is increased a point is soon reached where the yield ofv product recovered becomes less than would be obtained under milder iiashing conditions With no or less spray.

This diiiiculty is encountered in deep flashing .under severe conditions. It is encountered when the petroleum residue is heated in a choked heating coil, i. e. where sonic velocity is reached at orV near the exit of the heating Zone. It is especially encountered in deep iiashing operations when the residue is heated in an open coil,

i. e. the velocities are below sonic and when the heated ashing residue isv passed through a centrifugal separation zone, i. e. cyclone arrangement, to separate the flashed vapors from the unvaporized liquid residue.

It has been found under these conditions of deep flashing where clogging of the foraminous mist coalescer A reduced crude was vacuum flashed under severe condit-ions to produce catalytic cracking feed stock. The dashing was carried out in a commercial` vacuum flashing system in which system a wire mat `was employed with the `conventional overhead spray. The ligure, as will be ldiscussed below, illustrates the same apparatus modified to operate according to the` invention.

With a York mat six inches thick the'operation was started on the fifth of a month .and was satisfactory. However, by the thirteenth of the same month the entrainment had increased and it was necessary to reduce `the feed rate, and on the eighteenth -of the same month it was necessary to shut the plant down. It was found thatthe `mat was clogged over most of the area with from about 0.9 to 2.2 pounds of deposit per square foot of mat area. T he mat was therefore removed and operation continued at a reduced rate while holding the e11- trainment at a maximum of 0.1%.

Subsequently, a new mat was installed and the sprays were removed from above the mat and placed below the matas indicated in Figure l. With this arrangement continuous operation without clogging was achieved under Example :Il p In another case a commercial vacuum flasher with spherical ilashingechamber housing a centrifugal separation zone and equipped with a six inch wire mat sprayed from above was used to flash a reduced crude petroleum described above. In this case the severity of ilashing was somewhat less and operation over a period of about six months was possible although the pressure drop through the mat gradually increased from about 1/2 inch water to about 7 inches water during that time. This vacuum llasher was also modified according to the invention by removing the sprays from above the rnat and placing them so that the spray was downward counter to the rising vapors from a point just below the mat. With this arrangement continuous operation was possible with no measurable increase in the pressure drop.

An attempt has been made to explain the unexpected result. After operating the unit described in Example vII in the conventional manner and before modifying the unit as explained, this unit was studied while using only the spray without the mist coalescing mat and while maintaining the carry-over at a maximum of 0.1%. It was found that as the spray was increased from to about of the ilasher distillate the allowable feed rate could be increased while still holding the carry-over at the stated maximum value. For example, increasing the spray from 3040 B./D. to 3760 B./D. (about 13% to about 15%) allowed the feed rate to be increased by 1350 B./D. (i. e. from 33,000 to 34,350 B./D.). A further increase in the spray resulted in a loss in the percent by volume of flashed distillate recovered. It is evident from this that the downward rain of spray droplets through the ascending vapors tends to reduce the carryover of liquid residue. It appears therefore that when operating according to the invention the amount of material coalesced in the mat is reduced. Considering the great difference between the carry-over with and without the mat (0.10% and 0.01% however, it is evident that the mist coalescer still coalesces a large amount of material and without the spray oil washing the mat it would be expected to clog. A further discovery, however, appears to explain why under the conditions of this invention the clogging is not made worse but less. As previously explained the mist coalescer must be operated wet. This condition would normally be expected due to the mist which is continuously coalesced in the mat and drains therefrom. However, it is found that under deep hashing conditions the rising vapors are not at equilibrium with the liquid but are above their dew point. This is due at least in part to the centrifugal separation. The vapors therefore are capable of vaporizing further amounts of oil. They therefore tend to vaporize liquid eliminates the distillation, so to speak, of liquidv in the .mat. The mat therefore remains properly wet and the lluid coalesced material is able to drain from the mat thereby avoiding clogging.

Referring to the ligure, reduced crude introduced by line 1 and pump 2 is passed through the open coils of the heating furnace 3 wherein the material is partially vaporized and heated to a transfer temperature of, for example, 800 F. The transfer temperature is preferably the highest temperature to which the residue can be heated without any appreciable cracking, i. e. the incipient ,cracking tempera ure. This temperature is generally between about 750 and 820 F. A vacuum is pulled on line 4 by means of steam jets (not shown). The heated and partially vaporized reduced crude is then passed by the transfer line 5 to the centrifugal separator 6 built in the flash tower 7. Thus, the mixture enters the tower tangentially below the plate 8. The separated liquid descends rwill be between about 3% to the bottom section 9 where it is lightly stripped with steam. The vapors pass up through the center over-dow pipe 10 into the upper section 11 containing the six inch York mat 12 and the sprays 13. The liquid is sprayed downward with nozzles V,giving a solid cone spray pattern and the nozzles are so arranged over the cross section of the tower that the entire cross section of rising vapors is covered by the spray.V The pressure in the region above the mat may be from a few mm. Hg up to about 200 mm. Hg and in a typical case is around mm. Hg. Liquid coalesced in the mat and from the spray collects on the plate 8 and is withdrawn by line 21. In this particular case this liquid is combined with the unvaporized product in the bottom section 9. It may, however, be recycled to the heater if desired.

The vapors after passing through the mat 12 are withdrawn by line 4 to a condenser 14 wherein the major part of the product is condensed as llashed distillate. The condensate is removed by line 15. Part of it is passed by line 16 to feed the sprays. The vapors still remaining after passing through the condenser 14 and separator 17 are further cooled by a condenser 18 and are then exhausted by line 19. The material collected in separator 20 is essentially light gas oil.

The foraminous mist coalescer should have a large free volume and exert a low pressure drop as, for example, is the case in the mats mentioned above. It is, however, not essential that the packing be of metal wire as other materials such as glass wool and the like may be used. The coalescer is placed across the total area of vapor flow and may be of any thickness consistent with a suitably low pressure drop.

The amount of liquid sprayed counter to the ascending vapors will vary depending upon the severity of the flashing conditions and upon the percentage of the residual petroleum which is flashed but in any case should be enough to bring the vapor to its dew point, and usually and about 15%. Larger amounts can be used in some cases but larger amounts normally result in decreasing the yield of flashed distillate product.

Since the amount of flashed distillate which can be sprayed back into the flashed vapors without a substantial loss in yield of ashed distillate is quite limited and since the improvement according to the invention increases as the amount of spray is increased, the most advantageous results are obtained when all of the llashed distillate which is recontacted with the flashed vapors is introduced by the sprays just below the coalescer as described.

I claim as my invention:

1. In the method for the continuous recovery of catalytic cracking feed stock from petroleum residue which comprises passing petroleum residue through a heating zone the exit of which is maintained under sub-atmospheric pressure, heating the petroleum residue to incipient cracking temperature in said zone whereby the residue is largely but incompletely vaporized, passing the mixture of vapors and unvaporized liquid residue at subatmospheric pressure into a centrifugal separation zone wherein the vapors are separated from the unvaporized liquid residue, passing the vapors upward from the center of said centrifugal separation zone and through a foraminous mist coalescing zone, condensing the vapors after passing through said foraminous mist coalescing zone and spraying a portion of the resulting condensate in a downward direction through the rising vapors the improvement in which said condensate is sprayed at a point just below said formanious mist coalescing zone.

2. In the recovery of catalytic cracking feed stock from petroleum residue by Vacuum flashing comprising the steps of heating petroleum residue to incipient cracking temperature with partial vaporization under vacuum, centrifugally separating the vapors from the unvaporized residue in a centrifugal separation zone, passing the separated vapors through a foraminous mist coalescing zone said foraminous mist coalescing zone rather than above and then condensing them, and spraying part of the Said fOlaminOllS mist COaleSCing 2011. condensate-back into said vapors, the improvement which References Cited in the me of this patent comprises spraying said part of condensate in a downward direction counter to the vapors at a point just below 5 UNITED. STATES PATENTS 1,962,153 Peterkm June 12, 1934 

1. IN THE METHOD FOR THE CONTINUOUS RECOVERY OF CATALYTIC CRACKING FEED STOCK FROM PETROLEUM RESIDUE WHICH COMPRISES PASSING PETROLEUM RESIDUE THROUGH A HEATED ZONE THE EXIT OF WHICH IS MAINTAINED UNDER SUB-ATMOSPHERIC PRESSURE, HEATING THE PETROLEUM RESIDUE TO INCIPIENT CRACKING TEMPERATURE IN SAID ZONE WHEREBY THE RESIDUE IS LARGELY BUT INCOMPLETELY VAPORIZED, PASSING THE MIXTURE OF VAPORS AND UNVAPORIZED LIQUID RESIDUE AT SUBATMOSPHERIC PRESSURE INTO A CENTRIFUGAL SEPARATION ZONE WHEREIN THE VAPORS ARE SEPARATED FROM THE UNVAPORIZED LIQUID RESIDUE, PASSING THE VAPORS UPWARD FROM THE CENTER OF SAID CENTRIFUGAL SEPARATION ZONE AND THROUGH A FORAMINOUS MIST COALESCING ZONE CONDENSING THE VAPORS AFTER PASSING THROUGH SAID FORAMLINOUS MIST COALESCING ZONE AND SPRAYING A PORTION OF THE RESULTING CONDENSATE TO A DOWNWARD DIRECTION THROUGH THE RISING VAPORS THE IMPROVEMENT IN WHICH SAID CONDENSATE IS SPRAYED AT A POINT JUST BELOW SAID FORMATION MIST COALSCING ZONE. 